Arguably, the core principle of international law is, "everyone's got some slice of the planet, now let us all agree that past is past, and stick to the borders as they are now, so we can all focus on something more useful and mutually beneficial, instead of continuing to spend blood to adjust them". Attempts to change borders, or to create or remove countries, are quite a big deal.

Self-determination at the level of the existing countries is a core principle, not the individual: that's literally the reason we formed international law.